Lots of times ice dams occur not because the roof wasn't "raked" but because of issues with the attic space. Attics idealy need to be very well vented through the soffits and in the winter VERY COLD! An attic should preferably average the same temperature as the outside air. When it's this cold snow will not be as likely to hit the roof and begin melting which eventually forms an ice barrier. After $15,000 worth of ice damming issues two winters ago I became very educated in this subject.
Most folks attribute ice dams to the gutters backing up which does happen however a home can still have "damming" even without gutters installed.